Why is the <center> tag deprecated in HTML?

The <center> element was deprecated because it defines the presentation of its contents — it does not describe its contents.

One method of centering is to set the margin-left and margin-right properties of the element to auto, and then set the parent element’s text-align property to center. This guarantees that the element will be centered in all modern browsers.

According to W3Schools.com,

The center element was deprecated in HTML 4.01, and is not supported in XHTML 1.0 Strict DTD.

The HTML 4.01 spec gives this reason for deprecating the tag:

The CENTER element is exactly equivalent to specifying the DIV element with the align attribute set to "center".

What I do is take common tasks like centering or floating and make CSS classes out of them. When I do that I can use them throughout any of the pages. I can also call as many as I want on the same element.

.text_center {text-align: center;}
.center {margin: auto 0px;}
.float_left {float: left;}

Now I can use them in my HTML code to perform simple tasks.

<p class="text_center">Some Text</p>